PLDs are general-purpose integrated circuits that typically include both user-configurable circuitry and configuration control circuitry. The configuration control circuitry typically includes a configuration memory array and addressing circuitry for writing configuration data, which is typically transmitted in a bit stream, into the configuration memory array. The user-configurable circuitry typically includes logic elements and associated interconnect resources that are connected to the memory cells of the configuration memory array, and are programmed (configured) by the configuration data stored in the configuration memory array to implement user-defined logic operations (that is, a user's circuit).
One characteristic that distinguishes early PLDs from more recent PLDs is the ability to partially reconfigure the configuration memory array. Early PLDs included relatively simple configuration control circuitry that only supported full configuration of the configuration memory array (i.e., each time a change in the configuration was needed, the entire configuration memory array was rewritten). In addition, the memory cells of the configuration memory array were only accessible through the configuration control circuitry in these early PLDs. More recent PLDs include relatively sophisticated configuration control circuitry that allows partial reconfiguration of selected portions of the configuration memory array. Further, in contrast to the early PLDs, some memory cells of the configuration memory circuit in these more recent PLDs can be rewritten with data transmitted over the interconnect resources of the user-configurable circuitry.
The above-mentioned differences and other differences between early and recent conventional PLDs are explained in additional detail in the following sections.